DOCSIS 1.1 Upstream Speed config problem

Question

Helllo,
I am trying to configure a UBR7246 with IOS 12.3(23)BC4 to run 16QAM upstream channels with the intention of providing up to 3.Mbps of upstream bandwidth to my users.  Everything is working fine with 16QAM, minislot size of 4, and the "show cable modem QOS" command shows the upstream bandwidth and burst settings I have used in my modem config files.  I am using max burst of 4000 and max concatenated burst of 4000.
The problem is that I still cannot get over 2.10Mbps on an upstream with only 1 test modem on it and under 20% utilization on my 45Mbps connection to the Internet.  This is not an over-subscription issue or anything and I am wondering if I have something configured wrong.
Can anyone help me with how to get higher upstream bandwidth?  I know the theoretical max for this profile is 7.6Mbps, and I only want to do 3.0Mbps.  I know this is utilization depend, etc., but I only have one modem on the upstream.
I am only using cable upstream 3 for this and have edited out other parts of the config.
CMTS config items of interest:
interface Cable1/0
ip address x.x.x.x 255.255.255.192 secondary
ip address 10.203.240.1 255.255.248.0
ip access-group 110 in
ip access-group 110 out
no cable packet-cache
cable downstream channel-id 0
cable downstream annex B
cable downstream modulation 64qam
cable downstream interleave-depth 32
cable downstream frequency 117000000
no cable downstream rf-shutdown
cable downstream rf-power 50
cable upstream 3 frequency 28000000
cable upstream 3 docsis-mode tdma
cable upstream 3 channel-width 3200000
cable upstream 3 minislot-size 2
cable upstream 3 modulation-profile 9
no cable upstream 3 shutdown
cable arp filter request-send 3 2
cable arp filter reply-accept 3 2
cable source-verify dhcp
cable dhcp-giaddr policy
cable helper-address x.x.x.x
CMTS#show cable modem x.x.x.x qos
Sfid  Dir Curr  Sid   Sched  Prio MaxSusRate  MaxBrst   MinRsvRate  Throughput
          State       Type
15831 US  act   1083  BE     0    3000000     4000      0           2832
8810  US  act   1080  BE     4    0           3044      0           0
15838 DS  act   N/A   BE     0    10000000    1522      0           43
15850 DS  act   N/A   BE     4    0           3044      0           0
show cable modulation-profile:
9    request 16qam 128 no   0x0  0x10 0x152  0   8     no   yes   384   16qam na                        
9    initial 16qam 256 no   0x5  0x22 0x152  0   48    no   yes   256   16qam na
9    station 16qam 256 no   0x5  0x22 0x152  0   48    no   yes   256   16qam na
9    short   16qam 144 no   0x5  0x4C 0x152  7   8     yes  yes   368   16qam na
9    long    16qam 160 no   0x8  0xE7 0x152  0   8     yes  yes   352   16qam na
Modem config:
/* Style Definitions */
table.MsoNormalTable
{mso-style-name:"Table Normal";
mso-tstyle-rowband-size:0;
mso-tstyle-colband-size:0;
mso-style-noshow:yes;
mso-style-priority:99;
mso-style-qformat:yes;
mso-style-parent:"";
mso-padding-alt:0in 5.4pt 0in 5.4pt;
mso-para-margin:0in;
mso-para-margin-bottom:.0001pt;
mso-pagination:widow-orphan;
font-size:11.0pt;
font-family:"Calibri","sans-serif";
mso-ascii-font-family:Calibri;
mso-ascii-theme-font:minor-latin;
mso-fareast-font-family:"Times New Roman";
mso-fareast-theme-font:minor-fareast;
mso-hansi-font-family:Calibri;
mso-hansi-theme-font:minor-latin;
mso-bidi-font-family:"Times New Roman";
mso-bidi-theme-font:minor-bidi;}
        UsServiceFlow
                UsServiceFlowRef 1;
                QosParamSetType 7;
                MaxRateSustained 3000000;
                MaxTrafficBurst 4000;
                MaxConcatenatedBurst 4000;
                SchedulingType 2;
Thanks,
Bill

Answer 1

Thanks for the excellent responses!  I did resolve the problem and ended up using a combination of CMTS configuration and modem changes.  I did the following:
1.  Increased the upstream "max burst" and "max concatenated burst".  I tested with settings between 4000 and 16000.  The higher setting provided the largest bandwidth increase, (in the modem config), when coupled with a higher mini-slot setting.
2.  I used the "cable default phy burst" setting on the CMTS. made sure fragmentation was set at 2000, and turned off rate limiting on the upstream interface.
This is a small MSO with less than 5000 subscribers, and they are not prepared to move to DOCSIS 2 or 3.  I was able to use "mixed mode" for the modulation profile and provided them with upstream speeds of up-to 3.0Mbps.  (You can go to over 7Mbps if you want, but there are trade-offs in doing this.)
The new upstream speed tier is for a limited amout of customers and the MSO is quite happy now.
Let me know if anyone needs more info on this and I would be glad to provide it.  I imagine most folks have moved on to DOCSIS 2 and 3, but a lot of small MSO's cannot afford to do this.
Cheers,
Bill